Michelle Burger, who lives about 180 meters (196 yards) from Pistorius' house, said the screams woke her in the pre-dawn hours of Feb. 14 last year, when Pistorius killed Reeva Steenkamp by shooting four times through a toilet door.
Pistorius, 27, says he killed Steenkamp by mistake thinking she was a dangerous intruder. Prosecutors, however, say the world-famous athlete shot his girlfriend after a fight. As soon as the trial started they used Burger's testimony to hint at a loud argument before the fatal shots.
Burger's evidence contradicts Pistorius' version of events because the runner said he thought Steenkamp was in bed and did not describe any woman screaming.
"It was very traumatic," Burger said. "You could hear it was blood-curdling screams. You can't translate it into words. The anxiousness in her voice, and fear. It leaves you cold."
Burger said: "She screamed terribly and she yelled for help" and testified that she also heard a man shout for help before the shots were fired.
Pistorius' lawyer, Barry Roux, opened his cross-examination by asking Burger if she thought Pistorius was a liar. She didn't directly answer that but questioned Pistorius' version.
"I can only tell the court what I heard that evening," Burger said. "I cannot understand how I could clearly hear a woman scream but Mr. Pistorius could not hear it."
Roux, in an attempt to discredit her recollection of the sequence of events, argued that she heard just Pistorius screaming for help and trying to get into the bathroom by breaking down the locked door with a cricket bat.
Burger didn't concede ground, saying there was "no doubt" in her mind she heard two different people screaming, one a woman, and then the four gunshots, with a gap between the first shot and then the other three. She said she could not mistake gun shots for the sound of a cricket bat hitting a door.
The trial started 90 minutes late. Pistorius pleaded not guilty to charges of murder and three other counts relating to shooting guns in public in unrelated incidents and illegal possession of ammunition. Wearing a dark gray suit and black tie, he wrote in a pad and sometimes passed notes to his defense lawyers. At one point he smiled to a person sitting behind him.
The Olympian's murder trial is being broadcast live on TV in South Africa and across the world.
Defense lawyer Kenny Oldwadge laid out Pistorius' legal strategy, reading a statement from Pistorius in which he says the killing was an accident and that there were inconsistencies in the state's case.
Pistorius said he brought two fans in from the balcony on the night after speaking to his girlfriend who was in bed beside him. He said Steenkamp must have gone into the bathroom while he was fetching the fans. Pistorius said he did not notice that she had gone then heard the bathroom window open.
"I approached the bathroom, armed with my firearm, so as to defend Reeva and I," Pistorius said in the statement. He said he then heard a noise in the toilet cubicle and was in a "fearful state" because he was unable to run away or defend himself physically since he was not wearing his prosthetic legs. He said he shouted at what he thought was an intruder and then shot through the toilet door, only later realizing that he shot Steenkamp.
Reeva Steenkamp was a glamorous model and budding reality TV show star when she was cut down at age 29.
In the courtroom, Pistorius was seated near Steenkamp's mother, June. She was quoted in the Pretoria News from an interview she gave to a British newspaper saying that she wants to see him.
"I want to look at Oscar, really look him in the eyes, and see for myself the truth about what he did to Reeva," said June Steenkamp, 67. "Whatever the court decides at the end of his trial, I will be ready to forgive him ... But first I want to force him to look at me, Reeva's mother, and see the pain and anguish he has inflicted on me. I feel I need that."
At the start of the morning, showing the media intense interest in the trial, a drone carrying cameras flew over the entrance to the courthouse in gray, drizzly skies. Several broadcasters massed live broadcast satellite trucks around the courthouse. A 24-hour cable channel devoted to covering the trial was launched in South Africa on Sunday.
If convicted on the murder charge, Pistorius could be sent to prison for at least 25 years before the chance of parole, the minimum time someone must serve if given a life sentence in South Africa. South Africa does not have the death penalty.
Judge Thokozile Masipa, hearing the biggest trial of her career, will ultimately pronounce the champion runner innocent or guilty and will decide on any sentence. South Africa has no trial by jury.
As Pistorius left the courthouse, people erupted in hoots and a few boos.